Golf ball manufacturers typically are interested in determining various aerodynamic and mechanical characteristics for golf balls. One such aerodynamic characteristic that is critical to golf ball performance is the ball's spin decay rate during its flight. The spin decay rate indicates the change in spin rate over time. The greater the spin decay rate, the quicker the spin rate decreases. Consequently, the ball's trajectory is flatter, and the ball travels farther. In order to improve existing golf balls and develop better, new golf balls, manufacturers have used two techniques for measuring golf ball spin decay rates over its entire flight. One conventional technique uses a wind tunnel and the other technique uses a large testing range.
The first technique utilizes a golf ball that is supported on a spindle within the wind tunnel. The spindle is affixed to the surface of the ball. The spindle freely rotates on low friction bearings. With the ball immersed in the wind stream, a motor coupled to the spindle spins the ball to a predetermined rate. The motor is disconnected from the spindle, and the ball continues to spin under the influence of its inertia and spin decay rate. The ball's spin rate is monitored over time by a non-contact tachometer. The spin decay rate is determined by analyzing the spin rate versus time data.
This method is problematic for several reasons. First, there is a turbulence level present in the airstream of any wind tunnel, which is not present in the atmosphere through which a golf ball normally flies. Since a golf ball's aerodynamics are fundamentally turbulence driven, this can significantly affect the outcome of the test.
Second, in the wind tunnel the wind stream around the ball is disturbed at the point where the spindle is joined to the ball. This affects the spin decay measurement in a way for which it is difficult to compensate.
Furthermore, when using the wind tunnel, the ball's spin decay rate is determined by its aerodynamic spin resistance (i.e., drag) and the aerodynamic and mechanical spin resistance of the spindle and the bearings. As a result, the data must be corrected to account for the spin resistance of the spindle and bearings. Thus, the analysis of the data is more complex and the results obtained are less accurate.
A second technique to determine spin decay involves monitoring the entire golf ball flight trajectory continuously. This requires a testing range large enough to contain the ball's entire flight, which is can be 300 yards long. In the second technique, the ball is marked so that some but not all of its surface reflects a predetermined type of electromagnetic radiation source, such as light. The ball is launched into flight, and the source of radiation is directed at the ball. Sensors or cameras are set-up at spaced locations along the ball's flight path. Each sensor detects the pulses of energy radiation that are reflected off of the ball when the ball passes by the sensor. When a camera is used, time exposure images of the ball are taken along the path. When the reflective surface is facing the light source and camera, the camera image is an associated bright area. When the reflective surface is facing away from the light source and camera, the camera image is an associated dark area. As a result, the image of the ball's flight path is a series of alternating bright and dark areas called "broken-streak" images of the ball. The spin decay rate can be calculated by analysis of the resulting broken-streak images of the ball.
This method is troublesome for several reasons. First, testing ranges of sufficient size are typically outdoor ranges, since indoor ranges of the proper size are rare. Outdoor ranges are susceptible to uncontrollable environmental conditions, such as wind and lighting, which adversely impact the results or test schedule.
Furthermore, the distances between the sensors and the ball in flight are large. This requires either a large reflective marker on the ball or a very powerful electromagnetic radiation source. The large reflective marker can alter the ball's aerodynamic characteristics, which negatively affects the test results. The powerful radiation source has the drawbacks of being expensive, cumbersome, and possibly dangerous. In addition, the analysis of the broken-streak images is time consuming and the results are less accurate than desired.
It would therefore be desirable to provide a method and an apparatus for determining the spin decay rate of a golf ball using an indoor testing range and a ball in actual flight.